Field of the Invention
The present invention relates to a photodetector, to a method for fabricating such a photodetector, and to an image sensor.
Photodetectors are optronic components that are of key importance in many applications, among which digital photography and optical communications. The function of each photodetector is to detect a flux of electromagnetic radiation, and to convert this flux into an electrical quantity that may then be read by an electronic circuit.
Description of the Related Art
At present time, the most commonly used photodetectors are photodiodes fabricated in CMOS technology. However, photodiodes do not have sufficient selectivity with respect to the wavelength of the incident electromagnetic radiation, since it is sufficient for this wavelength to correspond to a photon energy that is higher than the bandgap of the semiconductor used, for the radiation to be detected. Furthermore, photodiodes do not have selectivity with respect to the polarization state of the electromagnetic radiation. For these reasons, when a detection of the radiation is desired, which allows different colours or different polarization states to be distinguished, each photodiode must be combined with a spectral filter or a polarizer. These additional optical components generate additional fabrication costs, and increase assembly complexity as they must be arranged with respect to the photodetectors, in particular within a colour image sensor.
Quantum photodetectors that are efficient in the spectral windows 3-5 μm (microns) and 8-14 μm are also known, but they need to be cooled to cryogenic temperatures, typically less than 77 K (kelvin). This cooling constraint limits or even prevents their use, especially in devices intended for the general public, and substantially increases their cost price.
Other types of photodetectors that are effective in the spectral band from 0.3 μm to 15 μm are also known, such as quarter-wave antennas, for example taking the form of slits or metal-insulator-metal structures. However the combinations of features that they have, between their detection-sensitivity level on the one hand and the width of their detection spectral interval on the other hand, are not suitable for applications such as imaging.
Moreover, Helmholtz electromagnetic-field resonators are known. Such a resonator comprises:                an electrically insulating volume; and        metal faces that surround the insulating volume along at least one looped path that forms a loop around this insulating volume, excepting two interruptions of the looped path, so that the metal faces form two electrodes that are separated from each other by at least one gap, called electric-field-concentrating gap and which contains the interruptions of the looped path.        
In such a Helmholtz resonator, the electric-field-concentrating gap has a thickness between both electrodes that is smaller than a thickness of the insulating volume, these thicknesses being measured in a common direction. Thus, when electromagnetic radiation is incident on the resonator, an electric field that is created by this radiation in the resonator is more intense in the electric-field-concentrating gap than in the insulating volume.
In addition, those skilled in the art know how to select the dimensions of the insulating volume and of the metal faces of such a resonator to produce a resonance of the electric field in the electric-field-concentrating gap, at a wavelength desired for the electromagnetic radiation.